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Experimental API support

Discussion of this proposal is held in this issue.

This proposal describes a mechanism that will allow library authors to provide experimental (that is, unstable or unfinished) API to their clients. An API is called experimental if, although publicly released as a part of the library, it may break at any moment and its usages will need to be recompiled. The proposed mechanism makes it possible to declare that the API is experimental, and to say “I understand that I'm using an experimental declaration that may break at any time” at the call site. Without such explicit consent from the user, a warning or an error is reported on usages of experimental APIs. The ultimate goal is to allow library authors to release APIs earlier and more frequently without fear of the necessity to support an incorrectly designed API for a long time because of source and/or binary compatibility.

Use cases in the standard Kotlin library

  • Annotations for type inference (@Exact, @OnlyInputTypes, etc.): we'd like to publish them, but don't want to commit to never changing semantics (see KT-13138, KT-13198)
  • Bitwise operations for integers
  • Coroutines-related APIs, where we're still experimenting a lot
  • Certain features for kotlin-reflect, that we're not sure about (see KT-15987, KT-15992)


  • Clear opt-in required for all users whose code may be broken by changes to the experimental API
  • Granularity: one should be able to mark a small part of an API so that only the users of that part are required to opt-in
  • Smooth graduation: if the API doesn't need to be changed as soon as it's released, just unmark it, and all clients remain binary compatible
  • Garbage collection upon graduation: when the API graduates, the opt-in flags/markers are highlighted as warnings (and subsequently, errors) so that clients know to remove them

Initial observations

  • One solution would be to use whole packages for experimental declarations, e.g. kotlin.experimental, kotlin.jvm.experimental, ... However, this is not granular enough and smooth graduation is not possible, because even if the API was completely fine from the start, it's going to be moved to a non-experimental package eventually, which will break binary clients.
  • A more natural solution would involve some explicit binary-retained annotation on each experimental declaration. This way, the compiler can check each call and each class usage, and report a warning/error if that symbol is experimental but no consent to using it has been given by the user. However, just one annotation isn't enough, because we'd like to force the user to opt-in to each experimental API (= group of declarations) separately. This is why we propose a design below where each experimental API declares its own marker annotation, which must be used at call sites.
    • We've explored the possibility of using a “string tag” argument instead of custom marker annotations (e.g. @Experimental(“kotlin.ExperimentalTypeInference”) ...) but discarded it because it's not as clean, involves more typing and reading, requires not to make typos, and complicates the implementation, especially in the IDE.
  • There's a number of ways to express the opt-in to use an experimental API, but a source-retained annotation (“local” opt-in) and a command line argument (“global” opt-in) would seem the most natural choices.


We propose to add the following declarations to the standard Kotlin library:

package kotlin

annotation class Experimental(val level: Level = Level.ERROR) {
    enum class Level { WARNING, ERROR }

annotation class UseExperimental(
    vararg val markerClass: KClass<out Annotation>

The Experimental annotation is applied to an annotation class and it makes that class an experimental API marker. There are two sets of use cases where markers are used:

  1. If a declaration is annotated with the marker, it becomes experimental under that marker and can use other experimental API with that same marker in its body.
  2. If a declaration or an expression is annotated with @UseExperimental(Marker::class), it can use other experimental API with the selected marker, but does not become experimental itself (its clients will not have to opt-in).

The first option of usage of experimental API markers is called a propagating opt-in (the annotation effectively causes propagation of the requirement to opt-in to the experimental behavior), and the second — a non-propagating opt-in. The user is free to choose whichever option is preferrable in each scenario.


// Library code:

annotation class ShinyNewAPI

// Class Foo is experimental API with marker ShinyNewAPI
class Foo { ... }

// Function bar is experimental API with marker ShinyNewAPI
fun = ...

// Usage:

// Function useFooAndBar uses experimental API with marker ShinyNewAPI and thus
// is required to opt-in to that marker. Here, we choose a propagating opt-in,
// because experimental API is used in the function signature (parameter foo's type).
// (But this is not enforced by the compiler in any way, we could've chosen
// a non-propagating opt-in as well.)
fun useFooAndBar(foo: Foo) {

// Function doSomething uses experimental API with marker ShinyNewAPI and is also
// required to opt-in. Here, we choose a non-propagating opt-in, because experimental
// API is used in the function body and it should not concern our clients
fun doSomething() {
    val foo = Foo()

Note that by using the experimental API and using the propagating opt-in, useFooAndBar effectively becomes experimental API itself (with the same marker). In theory, we could distinguish initial introduction of the experimental API and its propagating usages, but it would complicate the proposal a bit and there doesn't seem to be much value in doing that.

Both opt-in mechanisms allow to use the API for the selected markers anywhere in the parse tree lexically under the annotated element.

Using UseExperimental with annotations that are not experimental API markers has no effect and yields a compilation warning. (Note that this must not be an error because user code should not break once an annotation is no longer an experimental API marker.) Using UseExperimental with no arguments has no effect and yields a warning as well.

Propagation and opt-in for whole modules

Annotating every usage of an experimental API might quickly become annoying, especially for application modules, where the developer does not care about the clients of the code simply because application modules have no clients. In such cases, it'd be useful to have a module-wide switch to turn on (and propagate, if needed) the used experimental API.

We introduce two CLI arguments to kotlinc, which mirror the two ways to enable the experimental API:

  1., where is a fully qualified name of the experimental API marker, enables and propagates the corresponding API for the module. It's as if the whole module was annotated with
  2. enables the corresponding compile-time experimental API for the module. It's as if the whole module was annotated with @UseExperimental(

We recognize that a better way to handle this would be to introduce some kind of module annotations in the language, however the design of that feature is far from obvious and until it's done, we're proposing this quick intermediate solution instead.

Since it's not easy to encode arbitrary Kotlin expressions in the CLI arguments, and because experimental API markers are used in the -Xexperimental argument, we require all marker annotations to have no parameters. The compiler will report an error otherwise.

Note that using the propagating opt-in ( during the compilation will make the whole module “annotated” with the given experimental marker, i.e. all declarations in that module are going to be considered experimental with that marker. On JVM, this information is going to be stored in the .kotlin_module file. Upon checking call sites for experimental API usage, the Kotlin compiler is going to look not only at annotations on the declaration and its containing declarations, but also at the .kotlin_module of the containing module.

If any of the annotations mentioned in the -Xexperimental/-Xuse-experimental are deprecated, the compiler is going to report a warning or error, depending on the deprecation level.

The compiler will check the value of -Xuse-experimental in the same way it checks the argument of the @UseExperimental annotation.

Experimentality of Experimental/UseExperimental itself

Annotations Experimental and UseExperimental are proposed to be added to the standard library in Kotlin 1.3. Since we're not yet sure that this design is optimal, we would like to test it first, and see if we can finalize it. Therefore, we would like to keep this whole feature experimental itself, in the sense that we may change something incompatibly, and the client code must be aware of it.

Therefore, we will require each user of Experimental to provide at least one unstable (-X...) compiler argument, which would mean that the user is understanding the risks of using the experimental functionality. It can be either -Xexperimental=... or -Xuse-experimental=... with a specific marker, or the magic predefined argument -Xuse-experimental=kotlin.Experimental which doesn't allow using any experimental API by itself, yet merely allows using Experimental and UseExperimental in the source code. Unless one of these arguments is provided, the compiler will report a warning on each usage of Experimental or UseExperimental (but not on usages of the markers!).

Besides, we will also prohibit any usages of Experimental, UseExperimental and markers that do not aim to make use of the functionality declared in this proposal. The goal is to minimize the number of binary compatibility problems of user-compiled code if we decide to change something incompatibly. For example, you won't be able to use these classes as types:

// Error! Experimental cannot be used as a type
fun get(e: Experimental) = ...

In particular, this means that:

  1. Experimental and UseExperimental may only be used as annotations (but not as arguments to other annotations), as references in the import statement, or as qualifiers (to be able to access nested classes, e.g. Experimental.Level)
  2. Markers may only be used as annotations, as references in the import statement, or as a left-hand side to ::class literal in UseExperimental or WasExperimental (see below) arguments

Experimental and SinceKotlin in the standard library

For declarations in the standard library, as soon as a declaration is released, it'll have to be annotated with @SinceKotlin(X), where X is the earliest version, since which there have been no incompatible changes to the declaration. However, the -api-version compatibility argument will have no knowledge of how that declaration looked before it was released, i.e. the declaration will not be visible with -api-version Y for Y < X, even if it was present in the version Y and the opt-in was given by the user.

We don't intend to solve this problem completely because this would require us to know how the declaration looked in each release before it finally graduated (remember that experimental declarations can undergo binary-incompatible changes). To fix this at least partially, we'll add an internal standard library annotation WasExperimental:

package kotlin

internal annotation class WasExperimental(
    vararg val markerClass: KClass<out Annotation>

Usages of declarations annotated with WasExperimental are allowed even if the API version requirement is not satisfied, provided that the opt-in to all mentioned markers is given.

This feature allows us to release new standard library API in patch releases, further graduating it in a minor release. For example, suppose a function foo appears in the standard library as experimental in Kotlin 1.4.30. Since it's not yet graduated, it's not annotated with SinceKotlin:

// kotlin-stdlib 1.4.30
annotation class ExperimentalFooAPI

fun foo(s: String) {}

In Kotlin 1.5, the function is graduated (hence SinceKotlin(“1.5”)) and therefore is no longer annotated with ExperimentalFooAPI. To allow it to be used as experimental on 1.4.30 however, we also annotate it with WasExperimental so that for example the non-propagating opt-in -Xuse-experimental=ExperimentalFooAPI would work. (Of course, it also makes it possible to use it on 1.4.0...1.4.29, where there was no such function and linkage errors would arise, but we explicitly decide not to solve this problem.)

// kotlin-stdlib 1.5
@Deprecated("This experimental API has been released.")
annotation class ExperimentalFooAPI

fun foo(s: String) {}

Also note that upon graduation of the API, the experimental marker has been deprecated.

TODO: it's unclear at what point should the marker itself be annotated with SinceKotlin and with what value.

Other observations

  • Certain limitations for marker annotations arise:
    • Targets EXPRESSION and FILE are not possible for marker annotations, because these annotations operate on the declaration level, and neither expressions nor files are declarations in Kotlin. The compiler will report an error on the marker annotation if it declares one of these targets.
    • Marker annotations must have BINARY retention, otherwise the compiler will report an error. SOURCE retention is not enough because it wouldn't allow the compiler to read annotations from compiled code, and RUNTIME retention is not necessary because the fact that a declaration is experimental should not have any effect on how that declaration is visible at runtime through reflection.
    • As mentioned earlier, marker annotations must have no parameters, otherwise the compiler will report an error.
  • We've experimented with making experimental declarations “poisonous” in the sense that the requirement of the consent to use them propagates up the call chain, so that even indirect users of an experimental API would be aware of the fact that they're using something that may break at any moment, and would be required to opt-in by annotating their code or providing a command line argument. However, we found out that non-poisonous, or rather “poisonous to a certain degree” declarations are also required to fulfill our use cases, and their existence naturally leads to some sort of a classification of usages of experimental API into “signature usages” (those that have effect on the public API) and “body usages” (those that are used internally in function bodies). This leads to many other non-trivial questions, and overall complicates the design quite a bit, so we've decided simply to avoid mandatory propagation of experimental opt-in for now.

Known issues

  • Once the API has been released, its call sites are still using the marker annotation, which means that the annotation class will need to go through the deprecation cycle, which is somewhat inconvenient.